Colored silicon carbide

ABSTRACT

Silicon carbide particles coated with a pigmented coating result in silicon carbide particles which can be used in slip resistant flooring to achieve the desired slip resistance whilst giving improved aesthetic properties. The pigmented coating can be an organic coating such as a two part epoxy system or a water based or solvent based epoxy. Alternatively the pigmented coating can be an inorganic coating such as a ceramic glaze. Also provided are methods of coating silicon carbide particles with either an organic or an inorganic pigmented coating. Further provided are safety flooring materials including silicon carbide particles having a pigmented coating.

The present invention relates to the production of coated siliconcarbide particles, in particular to pigment coated silicon carbideparticles.

Silicon carbide, also known as carborundum is a synthetic solid materialcomprising silicon and carbon. The hardness of silicon carbide issimilar to that of diamond. Silicon carbide is produced electrolyticallyfrom graphite and silica under the influence of extreme heat and theproduct is then crushed to give a wide range of particle sizes.

Silicon carbide is a particulate material which has a smooth surfacemaking it reflective and angular edges making it useful in productsrequiring slip resistant properties. Silicon carbide is only availablein two colours which are black and dark green.

Silicon carbide is used in a variety of industries from grinding wheelmanufacture to flooring.

Silicon carbide, is used to confer slip resistance and wear resistanceto a variety of floors including vinyl safety floors. Its efficacy inachieving slip resistance has meant that its use is very wide spread inslip resistant flooring manufacture.

In flooring it is preferred to use the black version of the siliconcarbide although it is less pure.

Being black, and used on the surface of a safety flooring, siliconcarbide tends to darken the overall tone of the finished flooring. Thisis particularly an issue when high levels are used. In order to avoidthe tone being darkened excessively, attempts have been made to uselarger particles of silicon carbide, the idea being that, with largerparticles, for a given mass per unit area, fewer particles can be usedto achieve a particular slip resistance. However, with large blackparticles on a light background, the product appears very “functional”and is considered less aesthetically acceptable. Also if lost from thesurface, large particles leave large surface voids.

Some users dislike the high reflectivity of the silicon carbide which isconsidered undesirable for certain applications.

It would therefore be advantageous to provide silicon carbide particlesin colours other than black or dark green for use in slip resistantflooring to achieve the desired slip resistance while obtaining improvedaesthetic properties.

Accordingly from a first aspect the present invention provides siliconcarbide particles coated with a pigmented coating.

The coating may be a pigmented organic coating. Alternatively thecoating may be a pigmented inorganic coating.

Where the pigment is a pigmented organic coating it is preferably a twopart epoxy system where the two parts are mixed together to provide thepigmented organic coating.

Alternatively the pigmented organic coating may be a water based orsolvent based epoxy or two part solvent based polyurethane or waterbornepolyurethane or acrylic.

Where the coating is an inorganic coating it is preferably a ceramicglaze.

The silicon carbide particles may be coated with a coupling agentbeneath the pigmented coating to aid adhesion of the pigmented coating.

The coating of the silicon carbide particles with a pigmented resinallows a range of colours to be obtained whilst still maintaining theangular edges of the particles which provide the required slipresistance when used in safety flooring.

The pigment can be any colour and the colour can be solid or metallic.

The silicon carbide particle size range is preferably from 0.2 to 0.8mm.

According to a second aspect the present invention provides a method ofapplying an organic pigmented coating to silicon carbide particlescomprising the steps of:—

-   -   mixing silicon carbide particle with a liquid pigmented organic        coating system; curing the coated particles.

The coated particles are preferably sieved to remove any agglomerates.The organic system is preferably a two part system wherein the two partsare mixed together then mixing with the silicon carbide particles. Theorganic coating system is preferably applied at a rate of 90-140 g/m² ofsilicon carbide surface area, most preferably 120 g/m² silicon carbidesurface area.

The method preferably further comprises coating the silicon carbideparticle with a coupling agent before applying the organic coatingsystem.

The coupling agent is used to maximise adhesion of the organic coatingsystem to the silicon carbide particles.

More than one coating of the organic coating system can be applied

In a third aspect the present invention provides a method of applying aninorganic pigmented coating to silicon carbide particles comprising thesteps of fusing an inorganic pigmented coating onto the silicon carbideparticles.

The inorganic coating is preferably a ceramic glaze.

Preferably the ceramic glaze comprises:

-   -   1) silicon dioxide which promotes low expansion, high durability        and abrasion resistance;    -   2) aluminium trioxide which suppresses phase separation and        crystallization and improves acid resistance;    -   3) zirconia as an opacifier and to improve alkali resistance;    -   4) a selection of complex metal oxides as inorganic pigments;        and    -   5) a flux to reduce the temperature at which the components of        the glaze fuse together to form a homogenous mass. The flux        preferably includes combinations from alkali metal oxides,        alkaline earth metal oxides, zinc oxide, boric oxide and lead        oxide.

The alkali metal oxides may include lithium oxide, sodium oxide orpotassium oxide.

The alkaline earth metal oxides may include magnesium oxide, calciumoxide and barium oxide.

The glaze may be made by mixing and fusing the selected components in ahigh temperature furnace to form a glass. The glass is preferably milledto form a powder known as a frit.

To coat the silicon dioxide particles with the ceramic glaze the frit ispreferably applied to the particles, the water is removed by drying atalmost 100° C. and the coated particles are then fired at temperaturesof from 800° C. to 1400° C., most preferably 1200° C. to remelt thepowder and form a glaze.

The present invention further provides a safety flooring materialincluding the coloured silicon carbide particles of the presentinvention.

The silicon carbide particles confer slip resistance of the safetyflooring material owing to their angular edges.

The coloured silicon carbide particles may be the same colour as theflooring material or alternatively the coloured silicon carbideparticles may be a contrasting colour. The use of coloured siliconcarbide particles means that the desired aesthetic effects can beachieved.

The safety flooring material could be made from plasticised PVC,plasticised acrylic, rubber, epoxy or polyurethane flow applied resinsystems.

The present invention will now be described in more detail withreference to the following examples.

General Particle Coating

1. Organic Coating

The silicon carbide particles are mixed under low shear in a liquidpigmented organic coating, usually after coating with a coupling agentto improve adhesion of the final coating. The organic coating can be atwo component epoxy or alternatively it could be a water based orsolvent based epoxy or two part solvent based polyurethane or waterbornepolyurethane or acrylic.

The application of frictional heat and/or externally applied heatreduces the viscosity of the coating system, allowing good coveragebefore the onset of curing which is accompanied by a rise in theviscosity. With water based systems the water would first be removed at100° C.

Full curing is achieved after the coating has fully hardened onto thesilicon carbide and there is no evidence of residual coating liquid.

The resulting product is then sieved to give correct particle sizedistribution.

Particle agglomeration is avoided by optimising the ratio of coatingmaterial to silicon carbide and agitation by tumbling or stirring in arotary action mixer.

2. Inorganic Coating

Inorganic pigmented coatings such as vitreous enamel can be fused ontothe silicon carbide particle at high temperatures.

Specific Formulations and Method

1. Pre-treatment—The silicon carbide particles are coated with thefollowing coupling agent to maximise adhesion of the final coating. Thecoupling agent solution is added to the silicon carbide at 0.3 parts perhundred of silicon carbide and blended in a low shear rotary actionmixer before force drying if necessary.

Coupling agent formulation Parts by weight A1100 (gamma aminopropylsilane) 50 isopropyl alcohol (IPA) 50

2. Coating—A resin system is added to the dried, pre-treated siliconcarbide whilst mixing. Mixing is continued until the coating is cured.The product is forced dry and post cured if necessary.

The resin is added at a rate of 120 g per m² of silicon carbide surfacearea.

Resin System Formulations Parts by weight a) Solvent based epoxy Part A)Eurepox 7001/75 (75% epoxy in xylene) 420 Methyl isobutyl ketone 150Xylene 40 n-butanol 60 pigment/filler 330 Part B) Euredur 30/55(isolated polyamine adduct solution) 245 Xylene 20 Butanol 5 b) Waterbased epoxy Part A) Eurepox 776 (modified epoxy resin) 45 Pigment 16.5Barytes (barium sulphate) 38.5 Part B) Euredur XE36 (polyamine adduct inwater) 40 Water 10 c) Solvent free epoxy Part A) Eurepox 776 42 Pigment16 Barytes 41.6 Antifoam 0.4 Part B) Euredur 43 (a cycloaliphatic amine)25

In each two part resin system the whole of part A is mixed with thewhole of part B before application to the silicon carbide particles.Typically the particles are mixed with the two part resin system in aribbon or other rotary low shear mixer for about 60 minutes after whichtime the coating on the silicon carbide particles should be hard enoughto handle.

1. Silicon carbide particles coated with a pigmented coating. 2.Particles according to claim 1 wherein the coating is a pigmentedorganic coating.
 3. Particles according to claim 2 wherein the coatingis a two part epoxy system where the two parts are mixed together. 4.Particles according to claim 2 wherein the pigmented organic coating isa water based or solvent based epoxy.
 5. Particles according to claim 2wherein the pigmented organic coating is a two part solvent basedpolyurethane or waterborne polyurethane or acrylic.
 6. Particlesaccording to claim 1 wherein the coating is a pigmented inorganiccoating.
 7. Particles according to claim 6 wherein the coating is aceramic glaze.
 8. Particles according to claim 7 wherein the ceramicglaze comprises: silicon dioxide which promotes low expansion, highdurability and abrasion resistance; aluminium trioxide which suppressesphase separation and crystallization and improves acid resistance;zirconia as an opacifier and to improve alkali resistance; a selectionof complex metal oxides as inorganic pigments; and a flux to reduce thetemperature at which the components of the glaze fuse together to form ahomogenous mass.
 9. Particles according to claim 8 wherein the fluxincludes combinations from alkali metal oxides, alkaline earth metaloxides, zinc oxide, boric oxide and lead oxide.
 10. Particles accordingto claim 9 wherein the alkali metal oxides include lithium oxide, sodiumoxide or potassium oxide.
 11. Particles according to claim 9 wherein thealkaline earth metal oxides include magnesium oxide, calcium oxide andbarium oxide.
 12. Particles according to arty of claims 1-11, whereinparticles are coated with a coupling agent beneath the pigmented coatingto aid adhesion of the pigmented coating.
 13. Particles according to anyof claims 1-11 wherein the particles have a size range or from 0.2 to0.8 mm.